Tellurium finds immense application in the field of solar cells and alloys. Tellurium has been found to be the best additive for improving machinability in various types of ferritic steels (S. Wang, Journal of Metals, Vol. 63, 2011, p. 90). Although it is used in pigments, glass, lubricants, and rubber, the major application of tellurium is in solar cells (R. D. Brown, Minor Elements 2000, ed. Courtney Young, p. 307). Across the world, more than 90% of tellurium is produced from anode slimes collected from electrolytic copper refining and the remainder is derived from skimmings at lead refineries and from flue dust and gases generated during the smelting of bismuth, copper, and lead ores. The average world production of tellurium is estimated at 450 to 500 tonnes per year. Since anode slimes are the main resources for tellurium production, the estimation of reserves for tellurium is based on copper reserves. By applying a fixed recovery factor of 0.065 kg tellurium per tonne of copper, the world reserve of tellurium may be quoted as 34,000 tonnes.
In the process of electro-refining of copper, an impure copper anode is made and the anode which undergoes oxidation resulting in dissolution of copper in the electrolytic solution, which gets deposited on a cathode of copper metal or stainless steel. On the other hand, the associated impurities such as tellurium, selenium, precious group metals, etc. do not dissolve under the conditions prevailing in the electrolytic cell and settle at the bottom of the cell and are regarded as anode slimes (S. Wang et al. Journal of Minerals and Materials Characterization and Engineering, Vol. 2, 2003, p. 53). The copper anode slimes generated in the refinery unit vary widely in composition. They contain a significant amount of copper, lead, tellurium, selenium, precious group metals such as gold and silver along with a little amount of arsenic, antimony, bismuth, etc. (A. M. Amer, Physicochemical Problems of Mineral Processing, Vol. 36, 2002, p. 123). The aforementioned journal articles are hereby incorporated by reference herein.
The conventional processes implemented for the treatment of anode slimes are generally combinations of gyro- and hydrometallurgical techniques which cause undesirable emissions of selenium oxide, SO2, etc. leading to environmental pollution (U.S. Pat. No. 4,293,332). Hence, a process based only on hydrometallurgical unit operations is highly desirable from not only an economical point of view, but also from an environmental point of view.
While processing anode slimes through hydrometallurgical routes, the sequence of removing or recovering various metal values has been found to be the major concern for the metallurgists. Generally, the presence of a wide variety of impurities in the anode slime makes the process of recovering metal values complex. Specifically, if a large amount of lead is present in the anode slime, the recovery of tellurium, selenium, or precious metals becomes more difficult. A variety of pyrometallurgical techniques combined with acid leach processes have been attempted; however, they are not suitable due to the difficulty in removing significant amounts of lead from anode slime prior to the recovery of tellurium and/or precious group metals.
Reference is made to U.S. Pat. No. 4,352,786, wherein the anode slime is subjected to leaching in a medium of an ammonium acetate solution at a temperature not exceeding 80° C. After separating the leach solution from the undissolved slime residue, lead is crystallized from the leach liquor and recovered as lead acetate. However, the process possesses low efficiency of extraction of lead during ammonium acetate leaching, which is about 53%. Further, the use of ammonium acetate solution of 7 M concentration to achieve lead extraction of 53% may result in high chemical consumption.
Reference is also made to U.S. Pat. No. 4,283,224, wherein lead is separated by leaching the slime with diethylene triamine followed by carbonation to precipitate lead carbonate with subsequent regeneration of the leach solution. However, the process of removal of lead is complex in nature which involves the use of said amine to dissolve lead and subsequent recovery of the dissolved lead in the form of its carbonates through bubbling of carbon dioxide. In addition, although the lead removal step has been addressed, there is no information available about the feasibility of this process for processing anode slime containing tellurium.
Although various techniques have been proposed by several researchers to remove lead prior to the recovery of tellurium, none of them have been commercially successful.
Reference is made to U.S. Pat. No. 5,160,588, wherein the anode slime is leached with sulphuric acid and tellurium is recovered from the solution after leaching by suspending at least one copper electrode plate in a bath of a leach, allowing tellurium to deposit on the surface thereof as copper telluride. However, if a process would be available to recover tellurium as elemental tellurium, then it can provide higher economic value to the technology of processing anode slime for the extraction of tellurium. But the process mentioned here above (U.S. Pat. No. 5,160,588) recovers tellurium in the form of a compound, i.e. copper telluride.
Reference is made to U.S. Pat. No. 4,047,939, wherein the slime is leached with dilute sulphuric acid at an elevated temperature under oxygen partial pressure of up to 50 psi to dissolve copper and tellurium. The tellurium is then recovered as copper telluride from the leach liquor by adding metallic copper through cementation. However, the process involves autoclave leaching of anode slime at a pressure of 50 psi and a temperature of about 115° C. It would be advantageous if sulphuric acid leaching of anode slime can be operated at low temperature and atmospheric pressure. Further, this process also does not intend to recover tellurium as elemental tellurium. The aforementioned U.S. patents are hereby incorporated by reference herein.
Even though several methods have been proposed for the recovery of tellurium from anode slimes, none of them provide a process for the production of elemental tellurium rather than a tellurium compound.
It is therefore, highly desirable to develop a process that can be implemented for the significant separation of impurities before the recovery of tellurium from copper refinery anode slimes of any composition. Thus, the embodiments of the present invention provide a novel sequence of steps for separating metal values, where initially copper is removed followed by lead recovery; and finally, tellurium is produced from copper and lead free anode slime.